Patent documents 1 and 2 and non-patent document 1 disclose conventional electric double layer capacitors. Each of the conventional electric double layer capacitor includes a capacitor element and electrolytic solution impregnated in the capacitor element. The capacitor element includes two electrode units and a separator provided between the two electrode units. Each of the electrode units includes a collector made of metal foil, such as aluminum foil, and a polarizable electrode layer made mainly of activated carbon provided on the collector. Respective polarizable electrode layers of the two electrode units face each other across the separator.
The separator has the electrolytic solution impregnated therein to hold the electrolytic solution, and prevents an anode and a cathode from short-circuiting with each other. The separator contains cellulose having many hydroxyl groups to increase its hydrophilic property. The cellulose provides the separator with a large strength, a large flexible, an elasticity, and a hygroscopic property.
The electrolytic solution contains halogen, such as fluorine, as electrolyte to prevent the solution from deteriorating due to electrolysis.
In the conventional electric double layer capacitors, the separator may dissolve and have its strength reduced, accordingly reducing insulation between the electrode units.
Patent Document 1: JP10-256088A
Patent Document 2: JP2000-106327A
Non-Patent Document 1: “Aluminum Electrolyte Capacitor with Liquid Electrolyte Cathode” by Izaya Nagata, Japan Capacitor Industrial Co., Ltd. Feb. 24, 1997, 1st impression, 2nd edition, p. 368 to 373.